In order for that to work well, I have a hunch that they would need to do some very dirty tricks with register renaming, and add a 80-bit ("long double") mode t

In order for that to work well, I have a hunch that they would need to do some very dirty tricks with register renaming, and add a 80-bit ("long double") mode to the SSE hardware, and I suspect that's why they are not as fast as eachother (whether that's as a result of fully separate implementation, or very conservative microcode [possibly involving fencing and swapping registers, emulation of arithmetic quirks of x87]) in practice. Though, disclaimer: I am not even really an amateur at this level of analysis, so take what I've said with its very own salt lick.